The present invention relates to electronic control apparatuses. More specifically, the invention relates to an electronic control apparatus, such as a programmable controller, which contains device units including a power supply unit, a processing unit, input and output units, etc.
FIG. 17 is a perspective view showing the components of a conventional electronic control apparatus, i.e., a programmable controller (hereinafter referred to as "PC").
In FIG. 17, reference character A represents a box-shaped casing made of synthetic resin and having an open front; B, a power supply unit inserted into the casing A; and C, a processing unit adjacent to and narrower than the power supply unit B and including a CPU. The power supply unit B and the processing unit C are fixed to the casing A by screwing their front panels to the casing A. Reference character D represents a connecting portion provided in the processing unit C; E, a sequence program setting unit (hereinafter abbreviated as "setting unit"); and F, a connector from a printed circuit board in the setting unit E.
The setting unit E is attached to the front of the power supply unit B and the processing unit C by inserting its connector F into the connecting portion D. Front panels of two pairs of input units G and output units H are also screwed to the casing A at the side of the processing unit C. A terminal block I, to which a control object is to be connected, and a signal indicator portion J are provided on each of the input units G and the output units H. A printed circuit board K is attached to the back of each front panel, and is connected to a mother board (not shown) provided on the bottom of the casing A.
In the PC of the above construction, a control program preset by the setting unit E is stored in the processing unit C. An input signal from the control object is provided to the input unit G, subjected to processing by the processing unit C, and a control signal produced based on the processing results is output from the output unit H. As a result, the control object is sequence-controlled in accordance with the preset program.
In the conventional control electronic apparatus described above, the front panel of each device unit is fixed to the casing A by two or more screws. The apparatus is therefore cumbersome and requires numerous steps to initially assemble the apparatus and to disassemble and reassemble it at the time of maintenance or inspection. In addition, a large number of parts are needed. Further, where screws are directly engaged with the synthetic resin casing, the casing threads may be destroyed thus preventing proper assembly. Although this problem can be solved if molding is performed in order to employ nuts and bolts, this type of molding causes an additional molding cost.
Since the terminal blocks I are exposed from the front panels of the input units G and the output units H, there may occur not only accidental contact or short-circuiting but also a contact failure caused by the adherence of dirt, dust, etc. Also, since the input units G and the output units H have almost the same outward appearance, it is difficult to discriminate between the two, possibly leading to improper mounting/connection and/or faulty operation of those units, resulting in a fault of the apparatus. To prevent erroneous mounting/connection of the input and output units G and H, it has been proposed to indicate the type of each unit by characters, symbols, etc. provided on the front panel, or to form the front panels using resins of different colors for the respective types of front panels. However, these measures increase the number of steps and, therefore, the cost of the molding process. The setting unit E is attached to the front of the power supply unit B and the processing unit C by inserting the connector F into the connecting portion D. Variations in the height of the setting unit E after it is fixed to the apparatus may cause inconvenience in its handling, the existence of a step formed by the setting unit E and the front panels of the input and output units G and H is evidence of a poor design. The relative distance between the keys and the operator varies with the position of the operator, and it may therefore be the case that information on a display is misread due to light reflection at a certain viewing angle. If a program is erroneously set due to misreading of displayed information, a sequence control operation becomes abnormal, causing various problems. The control process may therefore have to be suspended to check the program.